Update create service principle command.

https://docs.microsoft.com/en-us/cli/azure/ad?view=azure-cli-latest

should be `az ad sp` create-for-rbac

Author: superyaniv

doc/source/microsoft/step-zero-azure.rst

@@ -168,7 +168,7 @@ If you prefer to use the Azure portal see the `Azure Kubernetes Service quicksta
 
    .. code-block:: bash
 
-      SP_PASSWD=$(az ad create-for-rbac \
+      SP_PASSWD=$(az ad sp create-for-rbac \
           --name <SERVICE-PRINCIPAL-NAME> \
           --role Contributor \
           --scopes $VNET_ID \